The influence of drought and natural rewetting on nitrogen dynamics in a coniferous ecosystem in Ireland

A field drought-rewetting treatment was artificially imposed in a Picea abies (L.) Karst. stand in each of four consecutive years (1992-1995) by means of a roof installed beneath the canopy. It was removed each year to allow natural rainfall rewet the soil in order to test Ulrich's (1980) 'Acidification pulse' hypothesis, viz. that rewetting a soil subsequent to drought results in net nitrification and acidification. NH₄⁺ concentrations in the Drought plot humus water increased substantially following drought and rewetting in 1992 (3754 μmol(c) L^-1, i.e. 36% total cations), to a lesser degree in 1994 and 1995, and were accompanied by elevated pH values (in the range pH 5.7 to 6.3), whereas NH₄⁺ levels and pH values (between pH 3.7 and 3.8) were unaffected during these times in the Control plot humus water. High NO₃^- concentrations were observed in the Drought humus water immediately after drought and rewetting in 1992, whereas in 1994 and 1995 there was a time-lag during which nitrate concentrations increased steadily. In contrast, NH₄⁺ and NO₃^- concentrations were consistently low at all depths in the mineral soil waters of both the Drought and Control plots throughout the years of the experiment. Likewise, pH values were unaffected in the mineral soil water at these times, suggesting that microbiological activity and nitrification was largely confined to the O horizon at this site. This research then, indicates that Ulrich's hypothesis was not applicable in the Ballyhooly site. Ammonification dominated over nitrification after drought and rewetting under current Irish climatic conditions. However, under conditions of increased air temperatures, as in the 'global warming' scenario, the potential for enhanced nitrification and consequent acidification and leaching of NO₃^- produced after drought and rewetting may exist at Ballyhooly.